SC4501EVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC4501EVB' PDF : 21 Pages View PDF
SC4501
POWER MANAGEMENT
Application Information
D
=
1
1
−
−
5
12 + 0.5
0.3
12 + 0.5
=
0.62
Therefore
ωp2
≤
2
(6.8Ω)• (10µF)
= 29.4Krads−1
= 4.68KHz
and
ωZ2
≥
6.8Ω • (1 − 0.62)2
4.7µH
=
209Krads−1
=
33.3KHz
The spacing between p2 and z2 is the closest when the
converter is delivering the maximum output current from
the lowest VIN. This represents the worst-case compensation
condition. Ignoring C5 and C6 for the moment, C4 forms a
low frequency pole with the equivalent output resistance
RO of the error amplifier:
RO
=
AmplifierOpen Loop Gain
Transconduc tance
=
49dB
60µΩ−1
=
4.7MΩ
1
1
ωp1 = − ROC4 = − 4.7MΩ • 820pF
= −260 rads−1 = −41Hz
C4 and R3 also forms a zero with angular frequency:
ωZ1
=
−
1
R3C4
=
−
1
30.9KΩ •
820pF
= −39.5 Krads−1 = −6.3KHz
The poles p1, p2 and the RHP zero z2 all increase phase
shift in the loop response. For stable operation, the overall
loop gain should cross 0dB with -20dB/decade slope. Due
to the presence of the RHP zero, the 0dB crossover frequency
should not be higher than
z2
3
. Placing z1 near p2 nulls its
effect and maximizes loop bandwidth. Thus
R3C4
≈
VOUTC2
2IOUT (MAX )
(15)
R3 determines the mid-band loop gain of the converter.
Increasing R3 increases the mid-band gain and the crossover
GND
R3
C4
C6
R2
SHDN
R1 C5
VOUT
C3 R4
U1
C1
C2
D1
L1
VIN
Figure 9. Suggested PCB Layout for the SC4501. Notice that there is no via
directly under the device. All vias are 12mil in diameter.
© 2005 Semtech Corp.
14
www.semtech.com
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